Function coeffs

pub fn coeffs(band: Band, f0: f32, q: f32, gain_db: f32, sr: f32) -> [f32; 5]

Normalised RBJ coefficients [b0, b1, b2, a1, a2] (a0 divided out). Shelves take Q like the peaks (Q = 0.707 is the classic slope-1 shelf).